Inductor-capacitor (LC) resonant circuits are known to be used to determine the proximity of one object to another. For example, in U.S. Pat. No. 5,661,470, entitled "Object Recognition System," issued to Karr, an LC resonant circuit tuned to a particular frequency is placed inside an object such as a toy. A base unit, which may include another toy, has an inductive loop that sends out electromagnetic energy pulses at the particular frequency such that the LC resonant circuit within the object responds to the base unit with energy at the same frequency only if the LC resonant circuit is sufficiently close to the base unit to receive the transmitted energy. If the LC resonant circuit responds to the base unit, the base units initiates a pre-established response, such as making a sound or turning on a motor. The described system is only sensitive to the proximity of the LC resonant circuit of the object with respect to the inductive loop of the base unit. That is, the system can identify whether or not the LC resonant circuit is within a certain distance from the base unit, but cannot further identify location information.
Karr also discloses systems which utilize multiple inductive loops in a base unit to better determine the location of an LC resonant circuit with respect to the base unit. In one example, multiple independent inductive loop antennas are arranged in a side-by-side format such that the inductive loop antenna that is closest to the LC resonant circuit will receive the strongest signal from the LC resonant circuit, thereby identifying the location of the LC resonant circuit with respect to the inductive loop antennas. In another example, multiple independent inductive loops are arranged in a matrix, creating a grid system in which strong signals are identified by both a row inductive loop and a column inductive loop, thereby pinpointing a particular location of the LC resonant circuit with respect to the matrix of inductive loop antennas. As described, both of these systems require multiple inductive loops in order to determine the location of an object relative to another object.
Inductive proximity systems implemented into electronic keyboards (either musical or alphanumeric) are known and disclosed in Karr and in U.S. Pat. No. 5,567,920, entitled "Position Reading Apparatus and Keyboard Apparatus," issued to Watanabe et al. (hereinafter Watanabe). Karr discloses a multiple frequency LC resonant circuit integrated into a keyboard that is manipulated to cause a base unit to play music. In Karr, the multifrequency LC resonant circuit is tuned to a particular frequency by activating a key. Each frequency in the LC resonant circuit corresponds to one keyboard key and provides one response per key. Likewise, Watanabe discloses a multiple frequency LC resonant circuit that is integrated into a keyboard with a one-key-to-one-frequency correspondence. As in Karr, depression of a key on the keyboard tunes the LC resonant circuit to a particular frequency with the tuned circuit resonating at a frequency that triggers a corresponding action to be taken by a receiving unit.
In the art of electronic percussion musical instruments, such as xylophones and marimbas, keys that are activated with force-sensing resistors or piezoelectric transducers are known. Force-sensing resistors indicate when a key is impacted, as well as the force with which the key is impacted, yet the force-sensing resistors have no sensitivity as to what area on the key is struck. For example, the force-sensitive resistors do not indicate whether a key is struck on the upper end of the key or on the lower end of the key. In addition, force-sensing resistors are incapable of responding to the mere presence of a mallet or to sustained contact between a mallet and a key.
Another electronic percussion instrument is disclosed in U.S. Pat. No. 4,980,519, entitled "Three Dimensional Baton and Gesture Sensor," issued to Mathews. The electronic drum of Mathews utilizes batons that actively transmit radio frequency signals to a position sensor. The batons are physically wired to the sensor system in order to drive the signals that are transmitted from the batons. While the electronic drum works well for its intended purpose, having batons that are tethered to a fixed object severely limits the expressive ability of a percussionist to freely manipulate the batons.
In view of the limitations involved with identifying a particular location using inductive sensors and with the limitations in the degrees of freedom provided by the keys of traditional force-sensitive keyboards, what is needed is an inductive sensor system that can expand the capability of key-based systems such as electronic musical instruments.